The invention relates to an air gap-insulated exhaust manifold for internal combustion engines.
Exhaust manifolds have the job of collecting the hot exhaust gases emerging from the cylinder exhaust ports of internal combustion engines and of conducting them to the downstream exhaust system components such as catalysts, mufflers, etc. The temperature of these gases subjects the manifold to high thermal loads. In an initial attempt to reduce the amount of heat discharged to the outside, exhaust manifolds were provided with water cooling systems. An example is described in DE 725 013 C1. In the development of this water-cooled exhaust manifold, special value was placed on the ability to compensate for the changes in the length of the various metal parts caused by the different degrees to which they are heated. This goal is achieved by the use of built-in sliding seats. Stuffing-box seals are provided at the points where the muffler brackets pass with play through the cooling jacket. These seals are designed so that any water which might leak out can escape only to the outside.
In recent times, exhaust manifolds have also been designed with air-gap insulation. A first example is described in EP 0 582 985 B. This exhaust manifold is formed by a flat flange, by an outer shell welded to the flange, and by an inner shell. The inner shell is welded to the outer shell and to the flange and is centered and held in place by wire rings in the outer shell. The inner shell is mounted in sliding seats located between the cylinder exhaust ports. The inner shell thus consists of a plurality of individual parts, which means that both production and assembly become more difficult and more expensive.
Another exhaust manifold is known from EP 0 671 551 A. This exhaust manifold consists of a flange, an outer shell, and an inner shell. The longitudinal edges of the outer shell are welded to the flange. The longitudinal edges of the inner shell rest loosely and elastically on the edges of the outer shell. No sliding seats or other means for compensating for the differences in length between the inner shell and the outer shell caused by different degrees of heating are provided.
Another air gap-insulated exhaust manifold is known from DE 196 28 797 C. This exhaust manifold has individual flanges for each cylindrical exhaust port, an outer shell, and an inner shell. The inner shell consists of a plurality of parts, and sliding seats are provided between the cylinder exhaust ports. In addition, a support cage is provided, which is positioned in the intermediate space between the inner and outer shells. This makes the exhaust manifold extremely complicated. The support cage also reduces the insulating effect of the air gap.
An air gap-insulated exhaust manifold with an inner shell consisting of two parts, a bottom with exhaust openings, and a loosely seated hood is known from DE 100 01 287 A. The inner shell is held in place by U-shaped straps, which are positioned in the air gap between the inner shell and the outer shell. The cross section of the outer shell is more-or-less omega-shaped. The outer shell, the retaining straps, the inner shell, and the flange are screwed to the engine block by way of an intermediate gasket. The disadvantage is the reduction in the insulating effect of the air gap caused by the retaining straps and by the large mounting surface on the engine block.